For a method of producing semiconductor devices by encapsulating semiconductor chips, such as IC or LSI, transfer molding with an epoxy resin composition is suitable as it is inexpensive and applicable to industrial mass production of the devices, and is widely employed in the art. The properties and the reliability of semiconductor devices are being improved by improving epoxy resin and its curing agent, phenolic resin.
However, with the recent market tendency toward small-sized, lightweight and high-performance electronic appliances, the degree of integration of semiconductors for such appliances is increasing year by year, and surface mounting of semiconductor devices is promoted. Given that situation, the prerequisites for the epoxy resin composition for use in encapsulating semiconductor chips are being much severer. Accordingly, there have occurred some problems that could not be solved by conventional epoxy resin compositions.
These days the materials for encapsulating semiconductor chips are required to have high flowability that is not detracted from by the inorganic filler to be added thereto in a large amount for improving the rapid curability thereof so as to improve the production efficiency and for improving the heat resistance and the reliability of encapsulated semiconductors.
An addition reaction product of a tertiary phosphine and a quinone that has good rapid curability is added as a curing accelerator to epoxy resin compositions for use in the filed of electric and electronic materials, for the purpose of accelerating the curing reaction of the curing resins (see, e.g., Patent Document 1).
Regarding the temperature range thereof, the curing accelerator of such a type may exhibit its curing acceleration effect even at relatively low temperatures. Therefore, though slightly, the curing reaction is accelerated even in the initial stage thereof, and because of this reaction, the resin composition shall have an increased molecular weight. The increased molecular weight thereof causes an increase in the resin viscosity, and, as a result, the resin composition that contains a large amount of a filler for reliability improvement may be problematic in that its flowability is poor and therefore its moldability is also poor.
For improving the flowability of resin compositions, various trials have heretofore been made of using a curability-suppressing component so as to protect the reactive substrate in the compositions. For example, a study has been made for protecting the active site of a curing accelerator with an ion pair to give a latency, and latent catalysts having a salt structure of an organic acid and a phosphonium ion are known (see, e.g., Patent Documents 2 and 3). However, since such ordinary salts contain the suppressing component therein all the time from the initial stage to the end stage of curing reaction, they could not attain sufficient curability though could attain flowability. That is, conventional latent catalysts could not satisfy both the two necessary requirements of flowability and curability at the same time.
Patent Document 1: JP 10-025335 A (page 2)
Patent Document 2: JP 2001-098053 A (page 5)
Patent Document 3: U.S. Pat. No. 4,171,420 (pages 2-4)